Why read old philosophy?

The irony is that you've internalized the revealed truths of your worldview to the point that you don't realize you have any. You don't believe that your philosophy and ethics make any assumptions (but they do). And when confronted the only response is that they're not assumptions, they're self-evident truths.

You're no better than a medieval European peasant in that regard.

First, I don't think any Christian would agree with you that "The Bible is a series of stories, genealogy and poetry, not an argument, or a dialogue that attempts to illuminate the truth, or at least points of disagreement over the truth." Of course the Bible is an illumination of truth -- that is its very foundation! The reason that the Abrahamic religions (Judaism, Christianity, and Islam) are also called the "revelatory" religions is that they are predicated on the idea that God reveals something true about the world, something that merely human knowledge would otherwise never be able to obtain. This is what makes religion so attractive in the first place -- it promises us a truth we could not know ourselves.

Second, this revelation of truth encompasses both the "why" and the "how," to use your terms. You are correct to say that it gives a story for "why" one might want to live a righteous life (i.e., to achieve salvation and be judged in heaven). But it also gives a explanation for "how" one should live such a life. The most obvious example of this is the Ten Commandments, but also consider other popular maxims like the ones contained in Jesus's Sermon on the Mount. Or think about how Aquinas writes an entire treatise about the differences between natural and divine law, and how the latter is given by God and instructs us on the proper way to live. The Church doesn't just make this stuff up out of thin air -- most of it is grounded in the Scripture.

So, finally, is the Bible so different from a text of ancient philosophy after all? You say that it is not remotely like the Nicomachean Ethics, but insofar as both teach something about both the "why" and the "how" to live a good life, I am not so sure. (To use your criteria of evaluation.) If you're unconvinced, perhaps consider either Plato's Republic or the Gorgias instead. In both those texts, Plato presents elaborate reasons for "why" to live in a particular way, but then concludes with an elaborate myth at the end (the Myth of Er and the Judgment of the Naked Souls). Why does he do this? Perhaps because he knows that, for many, reasoned arguments are not enough to convince. Something like a religion (i.e., a myth) is necessary to compel the masses into living in a way that is good for them. Is that so different from Christianity?

I'm so glad I took it that late. It was easily one of the best courses I ever took, of course with the professors to thank for that.

What did I get out of it? It got me thinking about all sorts of concepts, especially concepts I never would've thought about on my own. How do you quantify that? Who knows? I still think about Callicles from the Gorgias and how he'd observe some modern social phenomena and such.

But I think your posts are the sort of overfixation on "getting anything out if it" that the OP is talking about. It's a tempting question because it's usually unanswerable except in the obvious cases. But, for example, learning long division isn't helpful because you do it in the field (I haven't done it since school), rather it's helpful because you're exercising problem solving. Just like philosophy can exercise rationalism.

That they teach it at Eton is not a reason. At most a suggestion there could be a good reason for it.

To realize that ‘they were wise’ is not a reason. Why is that important to realize?

That there are ‘vast amounts of wisdom to be gleaned’ is a restatement, not an argument.

So whatever reading the old philosophers taught you, certainly not argumentation skills.